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A320445
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Primes p such that 2*p-1 and the concatenation of p and 2*p-1 are primes.
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1
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2, 367, 661, 691, 997, 1459, 2011, 2557, 2707, 3061, 3967, 4027, 4177, 4357, 4639, 5749, 6211, 6229, 6961, 7537, 7561, 7951, 8191, 8629, 8689, 9619, 10789, 10837, 11311, 12619, 13009, 13249, 13417, 13681, 14419, 14461, 14821, 15121, 15277, 15427, 15541, 15667, 15739, 15991, 16519, 17137, 17257
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OFFSET
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1,1
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COMMENTS
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All terms but the first == 1 (mod 6).
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LINKS
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EXAMPLE
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367 is in the sequence because 367, 2*367-1=733, and 367733 are all primes.
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MAPLE
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filter:= proc(p) local q, r;
if not isprime(p) then return false fi;
q:= 2*p-1;
r:= p*10^(1+ilog10(q))+q;
isprime(r) and isprime(q);
end proc:
select(filter, [2, seq(i, i=7..200000, 6)]);
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PROG
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(PARI) isok(p) = isprime(p) && isprime(2*p-1) && isprime(eval(concat(Str(p), Str(2*p-1)))); \\ Michel Marcus, Jan 10 2019
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CROSSREFS
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KEYWORD
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nonn,base
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AUTHOR
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STATUS
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approved
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